Maurizio Gaetani

Early Permian Pangea ‘B’ to Late Permian Pangea ‘A’

The pre-drift Wegenerian model of Pangea is almost universally accepted, but debate exists on its pre-Jurassic configuration since TedIrving introd ucedPangea ‘B’ by placing Gond wana farther to the east by V3000 km with respect to Laurasia on the basis of paleomagnetic data. New paleomagnetic data from radiometrically dated Early Permian volcanic rocks from parts of Adria that are tectonically coherent with Africa (Gondwana), integrated with published coeval data from Gondwana and Laurasia, again only from igneous rocks, fully support a Pangea ‘B’ configuration in the Early Permian. The use of paleomagnetic data strictly from igneous rocks excludes artifacts from sedimentary inclination error as a contributing explanation for Pangea ‘B’. The ultimate option to reject Pangea ‘B’ is to abandon the geocentric axial dipole hypothesis by introducing a significant non-dipole (zonal octupole) component in the Late Paleozoic time-averaged geomagnetic field. We demonstrate, however, by using a dataset consisting entirely of paleomagnetic directions with low inclinations from sampling sites confined to one hemisphere from Gondwana as well as Laurasia that the effects of a zonal octupole field contribution would not explain away the paleomagnetic evidence for Pangea ‘B’ in the Early Permian. We therefore regard the paleomagnetic evidence for an Early Permian Pangea ‘B’ as robust. The transformation from Pangea ‘B’ to Pangea ‘A’ took place during the Permian because Late Permian paleomagnetic data allow a Pangea ‘A’ configuration. We therefore review geological evidence from the literature in support of an intra-Pangea dextral megashear system. The transformation occurred after the cooling of the Variscan mega-suture andlasted V20 Myr. In this interval, the Neotethys Ocean opened between India/Arabia and the Cimmerian microcontinents in the east, while widespread lithospheric wrenching and magmatism took place in the west around the Adriatic promontory. The general distribution of plate boundaries and resulting driving forces are qualitatively consistent with a right-lateral shear couple between Gondwana and Laurasia during the Permian. Transcurrent plate boundaries associated with the Pangea transformation reactivated Variscan shear zones andwere subsequently exploitedby the opening of western Neotethyan seaways in the Jurassic.

Nature and evolution of Middle Triassic carbonate buildups in the Dolomites (Italy)

During the Middle Triassic, an evolution from carbonate bank to buildup, highly elevated in respect of the adjacent basins, is observed in the Alps. The Dolomites in the Southern Alps, Italy, are here considered as an example. During the Anisian, subtidal banks of the Upper Serla Fm., with rare peritidal cycles show only a limited biologic density and diversity. With the Late Anisian (Contrin Fm.) a gradual development of binding and framework-building communities can be observed. The sinking of wide areas of the Contrin carbonate bank into anoxic conditions (Plattenkalke Fm.) allowed the birth of isolated buildups. Their increased carbonate production, due to a greater diversity and density of organisms, was able to keep up with the rate of subsidence. The Latemar buildup — with additional data for the slope taken from Mt. Coldai (Civetta) and Sass da Putia — is used to illustrate the main character of a Lower Ladinian buildup. The restricted platform sediments display peritidal cycles: prevalent in the subtidal portion is Dasycladacean packstone/grainstone, associated with gastropods, ammonoids and blue-green algae; in the supratidal portion, desiccation and tepee structures are present. Scleractinian and sphinctozoan frameworks are rare on the upper part of the slope which is bound predominantly by encrusting organisms, mainly blue-green algae, porifers and Tubiphytes. The prevailing sediments on the upper part of the slope are bindstones, whilst packstones and rudstones are in the lower part of the slope. The latter carbonates were cemented early under submarine conditions, mostly by isopachous-palisade cement. Polyphasic internal sediments with phreatic-vadose influence are present, and can in the middle-lower part of the slope replace most of the original sediment.

The drift history of Iran from the Ordovician to the Triassic.

Abstract New Late Ordovician and Triassic palaeomagnetic data from Iran are presented. These data, in conjunction with data from the literature, provide insights on the drift history of Iran as part of Cimmeria during the Ordovician–Triassic. A robust agreement of palaeomagnetic poles of Iran and West Gondwana is observed for the Late Ordovician–earliest Carboniferous, indicating that Iran was part of Gondwana during that time. Data for the Late Permian–early Early Triassic indicate that Iran resided on subequatorial palaeolatitudes, clearly disengaged from the parental Gondwanan margin in the southern hemisphere. Since the late Early Triassic, Iran has been located in the northern hemisphere close to the Eurasian margin. This northward drift brought Iran to cover much of the Palaeotethys in approximately 35 Ma, at an average plate speed of c. 7–8 cm year−1, and was in part coeval to the transformation of Pangaea from an Irvingian B to a Wegenerian A-type configuration.

Tethyan oceanic currents and climate gradients 300 m.y. ago

We reconstruct the oceanic circulation pattern of the Tethys Ocean 300 m.y. ago by placing Late Carboniferous–Early Permian climate-sensitive biotic associations from Gondwana and Laurasia on a Pangea paleogeography constrained by selected paleomagnetic data. Warm-climate fossils and facies from Iran, located at that time along the Gondwanan margin of Arabia, are compatible with the existence in the Tethys Ocean of a warm subtropical surface current gyre, whereas cold surface currents swept the glaciated Gondwanan margin at higher southern latitudes, redistributing cold biota toward the tropics. This Tethyan surface current system and the associated narrow zonal barrier show similarities to recent glacial climate patterns. When placed on a large-scale paleogeographic reconstruction of Pangea of the B type, it neatly explains the otherwise problematic observation that the Carboniferous–Permian biota of Iran and northern Arabia is dominated by warm Euramerican and/or Russian taxa that are strikingly different from typical cold Gondwanan associations.

The Karakorum Block in Central Asia, from Ordovician to Cretaceous

Abstract The Karakorum Block records a predominantly marine Ordovician to Cretaceous sedimentary history. Six major sedimentary cycles are recognised. The oldest sediments, Early Ordovician, transgress over a crystalline basement. The 1-km-thick Ordovician-Silurian mostly shaly succession contains rare carbonate intercalations. In the Early Devonian, a wide peritidal platform spread over the craton. Sedimentation rates were low, 10–20 m/Ma, and Upper Devonian-probable Lower Carboniferous carbonates and clastics rest unconformably on the Early Devonian carbonates. No evidence has been found for either Late Carboniferous or glaciogenic deposits. Sedimentation rates increased during the Permian up to 50 m/Ma, with thicknesses between 1 and 2.5 km. Three steps are identified. (1) A huge alluvial to marginal-marine terrigenous prism aggraded during the Asselian-Sakmarian. (2) The Artinskian-Murgabian is characterised by local emergence and erosion, linked to extension with block rotations. (3) Greater differentiation occurred from Late Permian to Middle Triassic, when a peritidal carbonate flat developed in the southwest, facing a deeper basin to the northeast. Carbonate sediments prevailed throughout the Carnian-Norian. The Permo-Triassic evolution is interpreted as the passive margin stage of the Karakorum Block, which previously belonged to the Perigondwanian fringe, when, like Mega Lhasa, it drifted northward on the Tethyan Transit Plate. Mega Lhasa is considered as a collage of blocks possibly separated by thinned crust or short-lived seas with ocean crust. Quartzo-lithic sandstones with grains of mafic volcanics and serpentinite, overlain with gentle unconformity by Pliensbachian-Toarcian red sandstones with sedimentary and metasedimentary clasts, record the Eo-Cimmerian deformation in the Karakorum. This orogenic episode was over by the Aalenian, when a shallow-water carbonate ramp aggraded onto the previously emergent area. The pre-Barremian, Upper Jurassic and Lower Cretaceous are poorly documented. In post-Barremian times the sedimentary succession of N Karakorum was severely deformed in huge thrust sheets, including slabs of crystalline basement. A coarse conglomerate of mid-Cretaceous age sealed the thrust sheet edifice. Finally, isolated Campanian pelagic mudstones are recorded. Anticlockwise rotation of Mega Lhasa followed the Late Triassic docking of the Iranian Spur, using the latter as pivot. The rotation was completed during the Middle Jurassic, when SE Pamir, Shaksgam and Karakorum joined the Qiangtang, more closely assembling Mega Lhasa as it approached the Asian margin.

Southern Alpine lakes — Hypothesis of an erosional origin related to the Messinian entrenchment ☆

Abstract All the major lakes lying south of the Alps (Maggiore, Lugano, Como, Iseo and Garda) are cryptodepressions, unlike the northern Alpine lakes. Their glacial origin is discussed and refuted on the basis of geological, structural and geomorphic arguments. A major erosional phase is recorded south of the Alps after the deposition of the Gonfolite Formation (a synorogenic to postorogenic sedimentary wedge representing a deep-sea fan, ranging in age from Late Oligocene to Middle Miocene) and prior to the Pliocene transgression. The deposits of the Gonfolite have been deeply incised near Como: a tentative reconstruction of the inferred Messinian paleocanyon is proposed. The erosional surface can be followed under the Po Plain by means of seismic reflection profiles and is controlled by hundreds of deep exploratory wells. According to the available surface and subsurface data, the erosional phase can be dated at the Late Miocene (Messinian): it is related to the dramatic drop in sea level resulting from the isolation of the Mediterranean basin(s) from the world ocean at the close of the Miocene. The depressions presently occupied by the southern Alpine lakes are interpreted as canyons deeply incised along either former paleovalleys (as is the case for Lake Como) or along tectonically controlled embayments (as is the case for Lake Garda) during the evaporitic drawdown. The depressions created by the entrenchment have been eventually occupied and partly reexcavated by glaciers during the ice ages. Frontal moraines dammed the southernmost edge of the valleys, which are presently occupied by the lakes.

Unroofing history of Late Paleozoic magmatic arcs within the “Turan Plate” (Tuarkyr, Turkmenistan)

Abstract Stratigraphic, sedimentologic and petrographic data collected on the Kizilkaya sedimentary succession (Western Turkmenistan) demonstrate that the “Turan Plate” consists in fact of an amalgamation of Late Paleozoic to Triassic continental microblocks separated by ocean sutures. In the Kizilkaya area, an ophiolitic sequence including pyroxenite, gabbro, pillow basalt and chert, interpreted as the oceanic crust of a back-arc or intra-arc basin, is tectonically juxtaposed against volcaniclastic redbeds documenting penecontemporaneous felsic arc magmatism (Amanbulak Group). A collisional event took place around ?mid-Carboniferous times, when oceanic rocks underwent greenschist–facies metamorphism and a thick volcaniclastic wedge, with pyroclastic rocks interbedded in the lower part, accumulated (Kizilkaya Formation). The climax of orogenic activity is testified by arid fanglomerates shed from the rapid unroofing of a continental arc sequence, including Middle–Upper Devonian back-reef carbonates and cherts, and the underlying metamorphic and granitoid basement rocks (Yashmu Formation). After a short period of relative quiescence, renewed tectonic activity is indicated by a conglomeratic sequence documenting erosion of a sedimentary and metasedimentary succession including chert, sandstone, slate and a few carbonates. A final stage of rhyolitic magmatism took place during rapid unroofing of granitoid basement rocks (Kizildag Formation). Such a complex sequence of events recorded by the Kizilkaya episutural basin succession documents the stepwise assemblage of magmatic arcs and continental fragments to form the Turan microblock collage during the Late Paleozoic. Evolution of detrital modes is compatible with that predicted for juvenile to accreted and unroofed crustal blocks. The deposition of braidplain lithic arkoses in earliest Triassic time indicates that strong subsidence continued after the end of the volcanic activity, possibly in retroarc foreland basin settings. The occurrence of transgressive coquinas yielding endemic ammonoids (Dorikranites) characteristic of the whole Caspian area suggests proximity to the southern margin of the newly formed Eurasian continent in the late Early Triassic. The Late Triassic Eo-Cimmerian Orogeny caused only mild tilting and rejuvenation of the underlying succession in the study area. Only at this time were the Turan blocks, a series of Indonesian-type terranes comprised between the Mashad Paleo-Tethys Suture in the south and the Mangyshlak belt in the north, finally incorporated into the Eurasian landmass.

Pennsylvanian-Early Triassic stratigraphy in the Alborz Mountains (Iran)

Abstract New fieldwork was carried out in the central and eastern Alborz, addressing the sedimentary succession from the Pennsylvanian to the Early Triassic. A regional synthesis is proposed, based on sedimentary analysis and a wide collection of new palaeontological data. The Moscovian Qezelqaleh Formation, deposited in a mixed coastal marine and alluvial setting, is present in a restricted area of the eastern Alborz, transgressing on the Lower Carboniferous Mobarak and Dozdehband formations. The late Gzhelian–early Sakmarian Dorud Group is instead distributed over most of the studied area, being absent only in a narrow belt to the SE. The Dorud Group is typically tripartite, with a terrigenous unit in the lower part (Toyeh Formation), a carbonate intermediate part (Emarat and Ghosnavi formations, the former particularly rich in fusulinids), and a terrigenous upper unit (Shah Zeid Formation), which however seems to be confined to the central Alborz. A major gap in sedimentation occurred before the deposition of the overlying Ruteh Limestone, a thick package of packstone–wackestone interpreted as a carbonate ramp of Middle Permian age (Wordian–Capitanian). The Ruteh Limestone is absent in the eastern part of the range, and everywhere ends with an emersion surface, that may be karstified or covered by a lateritic soil. The Late Permian transgression was directed southwards in the central Alborz, where marine facies (Nesen Formation) are more common. Time-equivalent alluvial fans with marsh intercalations and lateritic soils (Qeshlaq Formation) are present in the east. Towards the end of the Permian most of the Alborz emerged, the marine facies being restricted to a small area on the Caspian side of the central Alborz. There, the Permo-Triassic boundary interval is somewhat similar to the Abadeh–Shahreza belt in central Iran, and contains oolites, flat microbialites and domal stromatolites, forming the base of the Elikah Formation. The P–T boundary is established on the basis of conodonts, small foraminifera and stable isotope data. The development of the lower and middle part of the Elikah Formation, still Early Triassic in age, contains vermicular bioturbated mudstone/wackestone, and anachronostic-facies-like gastropod oolites and flat pebble conglomerates. Three major factors control the sedimentary evolution. The succession is in phase with global sea-level curve in the Moscovian and from the Middle Permian upwards. It is out of phase around the Carboniferous–Permian boundary, when the Dorud Group was deposited during a global lowstand of sealevel. When the global deglaciation started in the Sakmarian, sedimentation stopped in the Alborz and the area emerged. Therefore, there is a consistent geodynamic control. From the Middle Permian upwards, passive margin conditions control the sedimentary evolution of the basin, which had its depocentre(s) to the north. Climate also had a significant role, as the Alborz drifted quickly northwards with other central Iran blocks towards the Turan active margin. It passed from a southern latitude through the aridity belt in the Middle Permian, across the equatorial humid belt in the Late Permian and reached the northern arid tropical belt in the Triassic.

The north Karakorum side of the Central Asia geopuzzle

An Italian geological team visited a remote and in part never studied area in the northern Hunza region (Pakistan), which represents the link between the Karakorum and Pamir Ranges. The north Karakorum sequence commences in the Permian with terrigenous sediments, followed by shallow- to deep-marine carbonates deposited on a newly formed passive margin. Deep-water sedimentation continued till the end of the Middle Triassic, when carbonate platform conditions resumed. An episode of deltaic red sandstones with orogenic provenance is interbedded in the Liassic, and it is transgressed by a Middle to ?Upper Jurassic shallow water marine unit. Eventually, all of the sequence was faulted and folded, with weak metamorphic imprint, before fluviatile red polygenic conglomerates sealed the succession,in a spectacular unconformity. The north Karakorum provides an example of a microplate that rifted away from Gondwana in the Permian, reached deep-marine conditions in the Early Triassic, and marginally recorded compressive movements in the Liassic. A subsequent orogenic episode points to a reorganization of the southern Asian margin possibly around middle Cretaceous time. Finally, the north Karakorum was affected by strong fold-thrust deformation and low- to very low-grade metamorphism in the Cainozoic, related to the India-Asia collision.

The geology of the Karakoram range, Pakistan: the new 1:100,000 geological map of Central-Western Karakoram

A new geological map of the central-western part of the Karakoram belt (Northern Areas and North West Frontier Province, Pakistan) is presented with its explanatory notes. The map is printed at a 1:100,000 scale, summarizing original field surveys performed at a 1:25,000 scale, which result from the first systematic reconnaissance of the area. This work represents the synthesis of several years of exploration studies and is mainly based based on original stratigraphic and structural field analyses focused on one of the less known orogenic belts of Central Asia. Original field surveys have been integrated within a GIS using georeferenced Russian topographic maps and grey-tone panchromatic SPOT images. The study area is located along the border between Pakistan and Afghanistan, extending from the top of the Chapursan Valley of the Hunza region to the Yarkhun Valley from the Karambar Pass to Gazin and to the upper part of the Rich Gol, which belong to Chitral. Three major tectonic units are exposed in the study area. From north to south they are: the East Hindu Kush-Wakhan, the Tirich Boundary Zone and the Karakoram Terrane. The first and the last units consist of Gondwana-related terranes showing a Pre-cambrian to earliest Paleozoic basement covered by Paleozoic to Mesozoic sedimentary successions which record their Late Paleozoic rifting from Gondwana, their drifting, and successive accretion to the Eurasian margin. They both show some similarities with the S-Parmir ranges, exposed to the north of the Afghan Wakhan. The Tirich Boundary Zone is a complex assemblage of high grade metabasites and gneiss with small remnants of sub-continental peridotites, which separate East Hindu Kush from the Karakoram. Its emplacement has been related to the possible opening of a basin between the two blocks at the end of the Paleozoic, followed by its deformation during the collision of Karakoram with East Hindu Kush, dating to the end of Triassic or beginning of the Jurassic. Detailed mapping has been carried out in the Karakoram belt, especially along its northern portion, which consists of a complex stack of tectono-stratigraphic units, showing peculiar stratigraphic and structural features. These units were progressively deformed and thrusted during the collision with the Kohistan Paleo-Arc and with India which occurred between the end of the Cretaceous and Paleogene. These collisions were also followed by continuous crustal thickening and by left-lateral shearing, which was especially active along the western margin of the mapped area. Our map also includes parts of the Karakoram Batholith, mainly Cretaceous in age, and of the Darkot-Gazin Metasedimentary Belt, which is exposed to the south of the main intrusive bodies and consists of Permo-Triassic metasediments.